1. Field of the Invention
The present invention relates to a stud bump structure, a package structure thereof and a method for manufacturing the package structure, and in particular, it relates to a silver alloy stud bump structure, a package structure thereof and a method for manufacturing the package structure.
2. Description of the Related Art
Advantages of flip chip assembly may include having many connection points, small distances between each connection point, small package areas, good performance at high frequencies, high reliability, and good resistance to electromagnetic interference. Therefore, flip chip assembly has been commonly used in packaging processes for an electronic device such as an integrated circuit (IC) or a light emitting diode (LED). Manufacturing and packaging a bump play an important role in the flip chip assembly process. Most of the flip chip bumps are made of solder alloys, such as Sn-37Pb, Sn-9Zn, Sn-0.7Cu, Sn-3.5Ag, Sn-51In, Sn-58Bi, Sn-3-Ag-0.5Cu, Sn-9Zn-3Bi, or the like. Methods for manufacturing a solder bump may include electroplating and stencil printing. However, a solder bump manufactured by electroplating is usually harmful to the environment and its specific alloy composition is usually difficult to control. In addition, it is also difficult to find an appropriate plating solution and plating process to form a Pb-free solder bump. For example, if a bump is formed of an alloy such as Sn-3.5Ag, Sn-0.7Cu, or Ag-0.5Cu, the composition of the alloy is usually hard to control. If a solder bump is to be formed of an alloy such as Sn-51In, Sn-58Bi, or Sn-9Zn—Bi, it is usually very difficult to find an appropriate plating solution.
Therefore, nowadays, stencil printing of solder paste has become an essential method in flip chip packaging processes. A key material for flip chip solder paste is solder powder. Generally, the particle size (diameter) of a solder powder in a surface mount technology (SMT) is between about 30 μm and 50 μm, wherein the solder powder of this size is easier to manufacture. However, since the size of a flip chip bump is usually smaller than 120 μm, the size of the solder powder is required to be smaller than 10 μm and the solder powder with this small size is very difficult to manufacture. In addition, when the size of the flip chip bump is decreased to be smaller than 100 μm, or even about 50 μm, each bump may only contain a few solder powders even if the size of the solder powder is smaller than 10 μm. Therefore, the difficulty of coplanarity tends to occur after a reflow process. Other problems in manufacturing a flip chip bump by solder paste include holes being formed by flux after a reflow process and manufacturing failures of the stencil printing may sharply increase when the distance between each connection point is less than 100 μm.
An electroplating gold bump or a gold stud bump manufactured by gold bond utilizing a gold wire can also be utilized as the flip chip bump for electrical connection. However, a problem of embrittlement at joint interface due to rapid formation of intermetallic compounds occurs when any of the electroplating gold bump and the gold stud bump is utilized as the flip chip bump. Further, if conventional soldering technologies are utilized for the assembly of substrate and any of the electroplating gold bump and the gold stud bump, a great quantity of gold from the electroplating gold bump or the gold stud bump will diffuse into the solder material, and a great quantity of brittle AuSn4 intermetallic compounds is formed due to the extremely rapid diffusion from gold into solder. As a result, bonding utilizing an electrically conductive adhesive is commonly the only way to assemble a chip and a substrate through the electroplating gold bump or the gold stud bump. One of the drawbacks from the adhesive bonding is provision of electrical conductivity worse than that provided by solder bonding. Further, the two great advantages provided by solder bonding, self alignment and reworkability, are no longer gained when utilizing adhesive bonding. Moreover, the electroplating gold bump and the gold stud bump are both expensive when considering the manufacture and material costs.
As a result, in the electronic packaging industry, an electroplating copper bump or copper pillar, or a copper stud bump manufactured by copper bond utilizing a copper wire are considered as an alternative material utilized in the solder bonding process. However, problems of floating solder at joint interfaces due to slow formation of intermetallic compounds potentially occurs when any of the electroplating copper bump, electroplating copper pillar and the copper stud bump is utilized as the flip chip bump. Moreover, copper tends to be oxidized and corroded, and therefore the reliability of the resulting package products is poor. More seriously, copper is too hard for the packaging technology, and under-pad chip cratering tends to occur during manufacture of the copper bumps, and a coplanarity problem also tends to occur during the assembly of the copper bumps and substrate. The coplanarity problem is further harmful in the ultrafine pitch packages and 3D-IC packages.